Israel last night dramatically escalated its war with Hamas, sending troops and tanks pouring over Gaza's borders in a move designed to reoccupy parts of the northern Gaza Strip. Amid reports of fierce clashes inside Gaza, columns of military vehicles and what the army said was "a sizeable number of troops" moved across the border at several points, backed by an intense air and artillery bombardment.

The move followed the failure of a week-long air force offensive, which has claimed more than 460 Palestinian lives, to halt the Hamas rockets. More than 30 hit Israel yesterday, wounding three people. Israel's defence minister, Ehud Barak, said his country was a peace-loving nation but Hamas had given it no choice and brought the assault on the Palestinian people. "Now is the time to do what needs to be done," he said. "It won't be easy. It won't be short. I don't want to delude anyone." The government in Jerusalem ordered the call-up of tens of thousands of reservists, suggesting the operation will be expanded further. The army said it expected to be in Gaza "for many long days".

France last night was swift to condemn the invasion, which it described as a dangerous military escalation that "complicated efforts by the international community to end the fighting, bring immediate aid to civilians and reach a permanent ceasefire". In London the foreign secretary David Miliband said the intensification of the Israeli assault would cause "alarm and dismay" and renewed calls for a swift cessation of violence. The UN Security Council also scheduled an emergency meeting to discuss the crisis, as the EU troika prepared to visit the region tomorrow.

Hamas broadcasts denounced the assault and said: "Gaza will be Israel's cemetery." The group claimed to have killed Israeli soldiers blown up in an armoured vehicle by a land mine, but there was no confirmation. Israeli forces claimed to have killed a number of Hamas fighters. Palestinian medics said at least one child and two other civilians were killed during the invasion.

As the ground incursion began, Israel jammed Hamas television broadcasts and replaced them with a caption: "Hamas leadership, your time is over."

The Israeli prime minister's spokesman, Mark Regev, described Hamas as "a totalitarian Taliban-like regime" but denied the intent of the assault was to remove Hamas from power.

The Israeli army said the invasion was intended to take control of territory from where Hamas fired its rockets. While it said Gazan civilians would not be targeted, a military spokesman added: "Anyone who hides a terrorist or weapons in his house is considered a terrorist."

"The objective is to destroy the Hamas terror infrastructure in the area of operations," said a spokeswoman, Major Avital Leibovitch.

"We are going to take some of the launch areas used by Hamas. We have many, many targets. In my estimation, it will be a lengthy operation."

Israeli forces also moved in to the Rafah area along Gaza's southern border with Egypt, where Hamas has used tunnels under the frontier to smuggle in weapons, including some of the rockets fired in to Israel. But the tunnels have also been used to bring desperately needed food and medicines into Gaza.

A spokesman for Palestinian president, Mahmoud Abbas, said he "vigorously condemned" the Israeli ground offensive. But Israel won the backing of the Czech government, which holds the EU presidency. Czech Prime Minister Mirek Topolanek said the Israeli assault was more "defensive than offensive".

But the invasion will be a blow to diplomatic efforts to win a truce. Hours before Israeli tanks crossed into Gaza, Gordon Brown called for an "immediate and urgent ceasefire" and for the territory's border with Egypt to be opened to allow in aid and for civilians to escape.

Brown is also understood to have appealed for Israel to step back from an incursion in a phone conversation yesterday with the Israeli leader, Ehud Olmert. Brown said there were issues on both sides to be tackled, including Hamas rocket attacks on Israel. Asked if he considered Israel's assault justifiable on the grounds of rocket attacks, Brown said: "I want an immediate and urgent ceasefire, but we want it to be based on deliverables for the future."

Last night UN Secretary-General Ban Ki-Moon added his voice to the condemnation and called for an immediate end to the ground operation. He was reported to have expressed his "extreme concern and disappointment" to Israeli Prime Minister Ehud Olmert in a telephone conversation.

Diplomats said that Tony Blair, envoy for the major powers sponsoring Middle East peace talks, is expected to hold discussions today with Israeli and Palestinian leaders, but not Hamas. The French President, Nicolas Sarkozy, is expected in Jerusalem tomorrow on a prearranged visit, but Israel has already rejected his call for a 48-hour ceasefire to allow in more humanitarian aid.

The exiled leader of Hamas, Khaled Meshaal, has rejected a ceasefire in Gaza until Israel agrees to end its three-year blockade of the territory, which has caused economic collapse and widespread hardship. "We will not break, we will not surrender or give in to your conditions, he said." Meshaal also warned that the organisation would defeat the Israeli ground assault.

The Israeli army did not say how deeply it intended to penetrate Gaza, but if it moves into built-up areas, it is likely to face strong resistance from Hamas, which has successfully destroyed armoured vehicles in the past, with political repercussions for Israel. But Israel said it was not the start of a total reoccupation of Gaza.

The ground offensive followed a day of heavy air, sea and artillery bombardment that left at least 18 people in Gaza dead, including children, and scores wounded. Twelve were killed when an Israeli missile strike hit a mosque in Beit Lahiya as worshippers were praying inside.

The death toll as the Israeli assault entered its second week rose to more than 460 Palestinians, about one third of them civilians or policemen, with four Israelis killed by Hamas rocket fire. An Israeli air strike also killed another senior Hamas official, Abu Zakaria al-Jamal. Israeli forces attacked the American school in Gaza, killing a guard. The Israeli military said the school was being used to store Hamas weapons and to shelter its fighters.